Titanium and its alloys are widely used in medicine as implants because of their biocompatibility and corrosion resistance. Furthermore, titanium alloys are good for the load-bearing applications because of high strength to weight ratio, high fatigue resistance, and relatively low, as for metals, Young's modulus. In addition, the Young's modulus and the associated stiffness of implant can by controlled by increasing porosity. There are many publications in which the positive impact of high roughness and porosity of the implant on the emergence of a strong connection between it and the bone is shown. The pore sizes of 100-400 microns is considered most preferred for biological implants, as it favors the penetration of the cells, tissue growth, vascularization, and nutrient transport.
The viability of titanium in the human body is determined to be around 20 years, but it could be extended up to twice, where the most common modification of the surface is with calcium-phosphate coating fabrication directly on titanium alloys. The calcium-phosphates coating on an implant provides a barrier, shielding tissues from possible release of ions from the titanium and other alloys. Furthermore, all calcium-phosphate ceramics are a good substrate as a structural support for cells, and cells proliferation. The most common calcium-phosphate ceramic used for implants is hydroxyapatite with chemical formula Ca10(PO4)6(OH2). Hydroxyapatite is the major inorganic mineral component of human bone, and numerous publications show that hydroxyapatite ceramic coating is the best promoter of proliferation of implanted cells, increases their survival, and improves their metabolism, when compared to the uncoated implant.
The biggest disadvantage of calcium-phosphate ceramics produced by engineering method is their low mechanical strength, which is revealed by the tendency to cracking and falling off of the implant shell fragments. The biggest disadvantage of the popular methods for producing hydroxyapatite coating on implants by thermal spraying is unfeasibility to use those methods when the entire volume of the porous implant has to be covered evenly.